The nature of the magnetic order in ${\left({\mathrm{La}}_{2/3}{\mathrm{Sr}}_{1/3}{\mathrm{MnO}}_3\right)}_9/\left({\mathrm{LaNiO}}_3\right){}_3$ superlattices is investigated using x-ray resonant magnetic reflectometry. We observe a $c$-axis magnetic helix state in the $\left({\mathrm{LaNiO}}_3\right){}_3$ layers that has not been reported in bulk nickelates, and which mediates the $\sim {130}^{\circ }$ magnetic coupling between the ferromagnetic ${\left({\mathrm{La}}_{2/3}{\mathrm{Sr}}_{1/3}{\mathrm{MnO}}_3\right)}_9$ layers, illustrating the power of x rays for discovering the magnetic state of complex oxide interfaces. Resonant inelastic x-ray scattering and x-ray absorption spectroscopy show that Ni-O ligand hole states from bulk ${\mathrm{LaNiO}}_3$ are mostly filled due to an interfacial electron transfer from Mn, driving the Ni orbitals closer to an atomiclike $3d^8$ configuration. We discuss the constraints imposed by this electronic configuration to the microscopic origin of the observed magnetic structure. The presence of a magnetic helix in $\left({\mathrm{La}}_{2/3}{\mathrm{Sr}}_{1/3}{\mathrm{MnO}}_3\right){{}_9/\left({\mathrm{LaNiO}}_3\right)}_3$ is crucial for modeling the potential spintronic functionality of this system and may be important for designing emergent magnetism in novel devices in general.

• Received 4 June 2018
• Revised 14 September 2018

DOI:https://doi.org/10.1103/PhysRevB.98.180401